1. Field of the Invention
The present invention relates to chemical process towers and, more particularly, but not by way of limitation, to a de-entrainment/mass transfer assembly for a tray tower, the trays having baffles, for increasing tray capacity and improving mass transfer efficiency therein.
2. Description of Related Art
It is a continuing goal to improve the efficiency of separation processes and like processes that are conducted using chemical process towers. To this end, many different approaches have been undertaken.
Separation processes that can be performed in chemical process towers include distillation and absorption. The optimum design of a tower having trays ensures maximum throughput (i.e. capacity) and mass transfer efficiency. At high throughput there is a tendency for liquid to be entrained at high gas velocity. This reduces the capacity as well as efficiency, both caused by the liquid blowing to the tray above. It is an objective of the present invention to effectively remove the entrainment.
Distillation and absorption towers are utilized to separate selected components from a multicomponent stream. Generally, such gas-liquid contact towers utilize either trays or packings, and sometimes combinations thereof. In the case of tray towers, any wetted solid surfaces will improve mass transfer through additional intimate contact between liquid and gas phases. There will be further advantage if the solid surface can also serve as a de-entrainment device.
Distillation trays come in two configurations: cross-current (cross-flow) and counter-current (dual-flow). The trays generally consist of a solid tray or deck having a plurality of apertures perforating the deck and are installed on support rings secured within the tower. In cross-current trays, gas ascends through the apertures and contacts the liquid moving across the tray through the “active” area thereof. It is in this area liquid and gas mix and fractionation occurs. The liquid is directed onto the tray by means of a vertical channel from the tray above. This channel is generally referred to as the inlet downcomer. The liquid moves across the tray and exits through a similar channel generally referred to as the exit downcomer. It is the active area of the tray which most directly effects gas liquid contact and thus mass transfer efficiency.
In the case of dual-flow trays, the tray deck covers the entire cross-sectional area of the tower. Gas and liquid flow through the same apertures, and contact in counter-current manner. Thus, there is no need for downcomers.
A problem common in the art is that of entrainment of spray in the gas. When spray is entrained in the rising gas it is carried to higher trays, thus affecting the composition of the mixture at those trays, with the consequence that the efficiency of separation of the components in the process mixture is compromised. The gas flow rate may be reduced in order to reduce the entrainment effect, but a consequence is that the tray throughput is also reduced. Another detrimental effect is that the entrained liquid accumulates on the tray above and there is increased amount of liquid on that tray, thus causing premature flooding or reduction in the capacity.
Improvements have been targeted for the technology of gas-liquid contact trays of the type discussed above to address throughput and mass transfer efficiency issues. Examples of this technology are seen in several prior art patents, which include U.S. Pat. Nos. 3,955,419, 4,604,247 and 4,597,916, each assigned to Glitsch, Inc. and U.S. Pat. No. 4,603,022 issued to Mitsubishi Jukogyo Kabushiki Kaisha of Tokyo, Japan. Other performance aspects are addressed in the prior art by the use of baffles, plates and de-entrainment devices. For example, U.S. Pat. Nos. 4,105,723 and 4,132,761, both assigned to Merricks Corporation, address special baffle and de-entrainment structures which are placed within a process tower.
Chuang et al. in U.S. Pat. No. 5,262,094 teach the utilization of a bed of packing material disposed beneath a fractionation tray for de-entraimnent. Further examples of de-entrainment performed using packing below trays are described by, for example, Monkelbaan et al. in U.S. Pat. Nos. 5,554,329 and 5,707,563, and by Nutter et al. in U.S. Pat. No. 5,975,504. Several other types of de-entrainment devices are described by, for example, Mahar in U.S. Pat. No. 4,274,923, Bentham in U.S. Pat. No. 4,818,346, Stober et al. in U.S. Pat. Nos. 5,837,105 and 6,059,934, and Ross et al. in U.S. Pat. No. 5,972,171. In another example, Lee et al. in U.S. Pat. No. 5,762,668 use a structured packing assembly to reduce entrainment in a chemical process tower and improve mass transfer efficiency.
However, when de-entrainment is performed using chevron types of demisting device or structured packings, the wetted surfaces of these devices are in direct contact with high velocity gas streams, resulting in re-entrainment of the liquid. This reduces the effectiveness of these devices for de-entrainment.